Methods of drawing high density nanowire arrays in a glassy matrix

ABSTRACT

The present invention provides a method of drawing a thermoelectrically active material in a glass cladding, comprising sealing off one end of a glass tube such that the tube has an open end and a closed end, introducing the thermoelectrically active material inside the glass tube and evacuating the tube by attaching the open end to a vacuum pump, heating a portion of the glass tube such that the glass partially melts and collapses under the vacuum such that the partially melted glass tube provides an ampoule containing the thermoelectric material to be used in a first drawing operation, introducing the ampoule containing the thermoelectric material into a heating device, increasing the temperature within the heating device such that the glass tube melts just enough for it to be drawn and drawing fibers of glass clad thermoelectrically active material. The invention further provides a method for bunching together such fibers and re-drawing them one or more times to produce arrays of thermoelectric nanowires clad in glass.

FIELD OF THE INVENTION

The present invention is directed to methods for producingthermoelectric devices and more particularly to methods of drawing highdensity nanowire arrays in a glassy matrix.

BACKGROUND OF THE INVENTION

Thermoelectric materials generate electricity when subjected to athermal gradient and produce a thermal gradient when electric current ispassed through them. Scientists have been trying to harness practicalthermoelectricity for decades because practical thermoelectricity could,inter alia: (1) replace fluorocarbons used in existing cooling systemssuch as refrigerators and air conditioners; and (2) reduce harmfulemissions during thermal power generation by converting some or most ofthe waste heat into electricity. However, the promise of practicalthermoelectricity has not yet been fulfilled. One problem is that,because of its low efficiency, the industry standard in thermoelectrictechnology cannot be functionally integrated into everyday heating andcooling products and systems.

Bulk form thermoelectric devices such as thermoelectric generators(TEG), thermoelectric refrigerators (TER) and thermoelectric heat pumpsare used for the direct conversion of heat into electricity, or for thedirect conversion of electricity into heat. However, the efficiency ofenergy conversion and/or coefficient of performance of these bulk formthermoelectric devices are considerably lower than those of conventionalreciprocating or rotary heat engines and vapor-compression systems. Inview of these drawbacks and the general immaturity of the technology,bulk form thermoelectric devices have not attained immense popularity.

Early thermoelectric junctions were fashioned from two different metalsor alloys capable of producing a small current when subjected to athermal gradient. A differential voltage is created as heat is carriedacross the junction, thereby converting a portion of the heat intoelectricity. Several junctions can be connected in series to providegreater voltages, connected in parallel to provide increased current, orboth. Modem thermoelectric generators can include numerous junctions inseries, resulting in higher voltages. Such thermoelectric generators canbe manufactured in modular form to provide for parallel connectivity toincrease the amount of generated current.

In 1821, Thomas Johann Seebeck discovered the first thermoelectriceffect, referred to as the Seebeck effect. Seebeck discovered that acompass needle is deflected when placed near a closed loop made of twodissimilar metals, when one of the two junctions is kept at a highertemperature than the other. This established that a voltage differenceis generated when there is a temperature difference between the twojunctions, wherein the voltage difference is dependent on the nature ofthe metals involved. The voltage (or EMF) generated per ° C. thermalgradient is known as Seebeck coefficient.

In 1833, Peltier discovered the second thermoelectric effect, known asthe Peltier effect. Peltier found that temperature changes occur at ajunction of dissimilar metals, whenever an electrical current is causedto flow through the junction. Heat is either absorbed or released at ajunction depending on the direction of the current flow.

Sir William Thomson, later known as Lord Kelvin, discovered a thirdthermoelectric effect called the Thomson effect, which relates to theheating or cooling of a single homogeneous current-carrying conductorsubjected to a temperature gradient. Lord Kelvin also established fourequations (the Kelvin relations) correlating the Seebeck, Peltier andThomson coefficients. In 1911, Altenkirch suggested using the principlesof thermoelectricity for the direct conversion of heat into electricity,or vice versa. He created a theory of thermoelectricity for powergeneration and cooling, wherein the Seebeck coefficient (thermo-power)was required to be as high as possible for best performance. The theoryalso required that the electrical conductivity to be as high aspossible, coupled with a minimal thermal conductivity.

Altenkirch established a criterion to determine the thermopowerconversion efficiency of a material, which he named the power factor(PF). The latter is represented by the equation: PF=S²*σ=S²/ρ, where Sis the Seebeck coefficient or thermo-power, σ is the electricalconductivity and ρ(1/σ) is the electrical resistivity. Altenkirch wasthereby led to establish the equation: Z=S²*σ/k=S²/ρ*k=PF/k, wherein Zis the thermoelectric figure of merit having the dimensions of K⁻¹. Theequation can be rendered dimensionless by multiplying it by the absolutetemperature, T, at which the measurements for S, ρ and k are conductedsuch that the dimensionless thermoelectric figure of merit or ZT factorequals (S²*σ/k)T. It follows that to improve the performance of athermoelectric device the power factor should be increased as much aspossible, whereas k (thermal conductivity) should be decreased as muchas possible.

The ZT factor of a material indicates its thermopower conversionefficiency. Forty years ago, the best ZT factor in existence was about0.6. After four decades of research, commercially available systems arestill limited to ZT values that barely approach 1. It is widelyrecognized that a ZT factor greater than 1 would open the door forthermoelectric power generation to begin supplanting existingpower-generating technologies, traditional home refrigerators, airconditioners, and more. Indeed, a practical thermoelectric technologywith a ZT factor of even 2.0 or more will likely lead to the productionof the next generation of heating and cooling systems. In view of theabove, there exists a need for a method for producing practicalthermoelectric technology that achieves an increased ZT factor of around2.0 or more.

Solid-state thermoelectric coolers and thermoelectric generators innano-structures have recently been shown to be capable of enhancedthermoelectric performance over that of corresponding thermoelectricdevices in bulk form. It has been demonstrated that when certainthermoelectrically active materials (such as PbTe, Bi₂Te₃ and SiGe) arereduced in size to the nanometer scale (typically about 4-100 nm), theZT factor increases dramatically. This increase in ZT has raisedexpectations of utilizing quantum confinement for developing practicalthermoelectric generators and coolers [refrigerators]. A variety ofpromising approaches such as transport and confinement in nanowires andquantum dots, reduction of thermal conductivity in the directionperpendicular to superlattice planes, and optimization of ternary orquaternary chalcogenides and skutterudites have been investigatedrecently. However, these approaches are cost-prohibitive and many of thematerials cannot be manufactured in significant amounts.

The ability to efficiently convert energy between different forms is oneof the most recognizable symbols of advances in science and engineering.Conversion of thermal energy to electrical power is the hallmark of theenergy economy, where even marginal improvements in efficiency andconversion methods can have enormous impact on monetary savings, energyreserves, and environmental effects. Similarly, electromechanical energyconversion lies at the heart of many modern machines. In view of thecontinuing quest for miniaturization of electronic circuitry, nanoscaledevices can play a role in energy conversion and also in the developmentof cooling technology of microelectronic circuitry where a large amountof heat is generated.

Accordingly, there exists a need for a broad spectrum of highperformance energy conversion and thermoelectric devices, based onone-dimensional inorganic nanostructures or nanowires.

There also exists a need for one-dimensional inorganic nanostructuresthat overcome deficiencies inherent in conventional thermoelectricdevices.

There further exists a need for a method for generating practicalthermoelectric devices from nanostructures that possess significantlylarger ZT factors as compared to those of thermoelectrically activematerials in bulk form.

In addition, there exists a need for a method for mass-producingpractical thermoelectric devices at a ZT factor of around 1.5 andhigher.

There further exists a need for a method for producing practicalthermoelectric devices that may be cost-effectively integrated intoeveryday heating and cooling products.

There also exists a need for a method for producing practicalthermoelectric devices that provide a smaller footprint than theindustry standard.

There further exists a need for a method for producing practicalthermoelectric devices capable of being mass-produced at a lower costthan the current industry standard.

In addition, there exists a need for a method for generating electricpower from thermoelectric generators to utilize waste heat (e.g.,industrial, domestic, automobile, etc.).

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a method for generating practical thermoelectric devices fromnanostructures that possess significantly larger ZT factors as comparedto those of thermoelectrically active materials in bulk form.

It is an additional object of the present invention to provide a methodfor mass-producing practical thermoelectric devices at a ZT factor ofaround 1.5 and higher.

It is another object of the present invention to provide a method forproducing practical thermoelectric devices that may be cost-effectivelyintegrated into everyday heating and cooling products.

Additionally, it is an object of the present invention to provide amethod for producing practical thermoelectric devices that provide asmaller footprint than the industry standard.

It is a further object of the present invention to provide a method forproducing practical thermoelectric devices capable of beingmass-produced at a lower cost than the current industry standard.

It is yet another object of the present invention to provide a methodfor generating electric power from thermoelectric generators to utilizewaste heat (e.g., industrial, domestic, automobile, etc.).

The present invention pertains to nanostructures formed from fibers ofthermoelectrically active materials that are substantiallyone-dimensional, having a diameter that is significantly smaller thantheir length. The fibers from which these nanostructures are composedhave a diameter of approximately 200 nm or less. The inventivenanostructures described herein are referred to as, “nanowires”,cables”, “arrays”, “heterostructures” or “composites” that contain aplurality of one-dimensional fibers. The cables preferably comprise atleast one thermoelectrically active material and a glassy material,which acts as an electrical insulator for the thermoelectrically activematerial, which is also referred to herein as the “thermoelectricmaterial”.

According to another aspect of the invention, the thermoelectricmaterial comprises a large concentration (e.g., 10⁶-10¹⁰/cm²) ofnano-sized wires embedded in a suitable glass forming a cable, whereinthe thermoelectric material is in the form of a glass-clad nanowirescomprising a plurality of one-dimensional fibers that extend over largedistances along the length of the cable without coming in contact withother fibers. The thermoelectrically active material may comprise asuitable metal, alloy or semiconductor material, which maintains theintegrity of the interface between the thermoelectric material and theglassy material without any appreciable smearing and/or diffusion of thethermoelectric material.

According to a further aspect of the invention, a process forfabricating cables includes increasing the population of thermoelectricfibers to more than 10⁹/cm² of the cross-section of the cable. Eachcable includes an array of fibers having a distribution of diameters,wherein the variation in fiber diameter may be reduced by employingautomated draw-towers, which are commonly employed in the fiber-opticindustry for drawing optical fibers.

A preferred cable produced in accordance with the principles of thepresent invention preferably comprises at least one thermoelectric fiberembedded in an electrically insulating material, wherein thethermoelectric material exhibits quantum confinement. The preferredcable comprises a plurality of fibers such that there is electricalconnectivity between the ends of all the fibers. Alternatively, there iselectrical connectivity between some, but not all of, the fibers of thecable. The glass cladding for the cable preferably comprises anelectrically insulating material such as pyrex, borosilcate,aluminosilicate, quartz or a glass having lead oxide, tellurium dioxideand silicon dioxide as its main constituents. The thermoelectricmaterial may be chosen from the group consisting of a metal, asemi-metal, an alloy and a semiconductor, such that the thermoelectricmaterial exhibits electrical connectivity and quantum confinement.

The present invention also provides a method of drawing athermoelectrically active material in a glass cladding, comprisingsealing off one end of a glass tube such that the tube has an open endand a closed end, introducing the thermoelectrically active materialinside the glass tube and evacuating the tube by attaching the open endto a vacuum pump, heating a portion of the glass tube such that theglass partially melts and collapses under the vacuum such that thepartially melted glass tube provides an ampoule containing thethermoelectric material to be used in a first drawing operation,introducing the ampoule containing the thermoelectric material into aheating device, increasing the temperature within the heating devicesuch that the glass tube melts just enough for it to be drawn anddrawing fibers of glass clad thermoelectrically active material. Themethod may further comprise bunching the fibers of glass cladthermoelectrically active material together and redrawing one or moretimes in succession to produce a multi-core cable having a plurality ofindividual thermoelectric fibers that are insulated from each other bythe glass cladding.

Additionally, the above-described method may further comprise the stepsof breaking the glass clad fibers into shorter pieces, introducing thepieces of glass clad fibers into another glass tube having a sealed endand an open end, evacuating the tube by attaching the open end to avacuum pump, heating a portion of the glass tube such that the glasspartially melts and collapses under the vacuum such that the partiallymelted glass tube provides an ampoule containing the pieces of glassclad fibers, introducing the ampoule into a heating device, increasingthe temperature within the heating device such that the glass tube meltsjust enough for it to be drawn and drawing fibers of glass cladthermoelectrically active material to produce a cable having a pluralityof multi-core fibers.

Further objects and advantages of the invention will be brought out inthe following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a tubular furnace for drawing athermoelectrically active material embedded in a glass cladding, inaccordance with the principles of the present invention;

FIG. 2 is an x-ray diffraction pattern of a PbTe-based cableconstructed, in accordance with the principles of the invention;

FIG. 3 is a side view of a glass-clad PbTe-based cable constructed inaccordance with the principles of the invention;

FIG. 4 is an enlarged cross-sectional view of the glass-clad PbTe-basedcable of FIG. 3 taken along line 3A-3A;

FIG. 5 is a cross-sectional view of the glass-clad PbTe-based cable ofFIG. 3 after a second drawing of the PbTe fibers;

FIG. 6 is a cross-sectional view of the glass-clad PbTe-based cable ofFIG. 3 after a third drawing of the PbTe fibers;

FIG. 7 is a chart illustrating the DC resistance of the PbTe cable ofFIG. 4 (after a first drawing of the PbTe fibers);

FIG. 8 is a chart illustrating the DC resistance of a PbTe cable of FIG.5 (after a second drawing of the PbTe fibers); and

FIG. 9 is a chart illustrating the DC resistance of a PbTe cable of FIG.6 (after a third drawing of the PbTe fibers).

DETAILED DESCRIPTION

In the following paragraphs, the present invention will be described indetail by way of example with reference to the attached drawings.Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars, rather than as limitations on thepresent invention. As used herein, the “present invention” refers to anyone of the embodiments of the invention described herein, and anyequivalents. Furthermore, reference to various feature(s) of the“present invention” throughout this document does not mean that allclaimed embodiments or methods must include the referenced feature(s).

Before starting a description of the Figures, some terms will now bedefined.

Bulk Material: Macroscopic-sized thermoelectric materials that aretypically larger than 1 micron or 1 micrometer in all three dimensions.

Chalcogenides: Group VI elements of the periodic table.

Chemical Vapor Deposition: Deposition of thin films (usuallydielectrics/insulators) on wafer substrates by placing the wafers in amixture of gases, which react at the surface of the wafers. This can bedone at medium to high temperature in a furnace, or in a reactor inwhich the wafers are heated but the walls of the reactor are not. Plasmaenhanced chemical vapor deposition avoids the need for a hightemperature by exciting the reactant gases into a plasma.

Doping: Deliberately adding a very small amount of foreign substance toan otherwise very pure semiconductor crystal. These added impuritiesgive the semiconductor an excess of conducting electrons or an excess ofconducting holes (the absence of conducting electrons).

Efficiency: Efficiency is the power generated by a system divided by thepower fed into it, a measure of how well a material converts one form ofenergy into another. Efficiency stands at a mere 8 to 12% for bulk formthermoelectric devices that are currently available or on the nearhorizon.

Figure of Merit: The thermoelectric figure of merit, ZT, is given byZT=(S²*σ/k)*T, where S is the Seebeck coefficient, T is the absolutetemperature, a is the electrical resistivity, and k is the thermalconductivity.

Lead Telluride: PbTe is one of the most commonly used thermoelectricmaterial other than Bi₂Te₃. PbTe is typically used for power generationbecause this material exhibits its highest ZT at temperatures between400 and 500° C. and has an effective operating range of about 200° C.around 500° C.

Nano: A prefix meaning one-billionth, or 0.000000001. For example, thewavelength of the ultraviolet light used to etch silicon chips is a fewhundred nanometers. The symbol for nanometer is nm.

Quantum Confinement: Quantum Confinement takes place when carriers ofelectricity (electrons or holes) are confined in space by reducing thesize of the conductor. For example, a very thin conducting film reducesthe freedom of a carrier by limiting its freedom to propagate in adirection perpendicular to the plane of the film. The film is said to bea 2-d structure and the carrier in such a film is said to be quantumconfined in one direction. Carrier transport can take place in the twodirections available in the plane of the film. In a nanowire, quantumconfinement can occur in two directions and the only direction availablefor carrier transport is along the length of the wire.

Seebeck Coefficient: The electromotive force generated in a materialwhen it is subjected to a thermal gradient and is normally expressed asmicrovolts per degree Kelvin. The thermoelectric power, or Seebeckcoefficient, of a material has a large role in determining its ZTfactor.

Thermal Conductivity: Thermal conductivity is an inherent property of amaterial that specifies the amount of heat transferred through amaterial of unit cross-section and unit thickness for unit temperaturegradient. Though thermal conductivity is an intrinsic property of amedium, it depends on the measurement temperature. The thermalconductivity of air is about 50% greater than that of water vapor,whereas the thermal conductivity of liquid water is about 25 times thatof air. Thermal conductivities of solids, especially metals, arethousands of times greater than that of air.

The present invention is directed to nanostructures referred to hereinas “nanowires”, “cables”, “arrays”, “heterostructures” or “composites”that contain a plurality of one-dimensional fibers. Nanowires inaccordance with the present invention generally compriseheterostructures of at least one thermoelectrically active material andone other compositionally and structurally different material (e.g.,glass), wherein an interface or junction is formed therebetween. Thethermoelectrically active material is reduced in thickness or diameterto nano-dimensions in order to harness the advantages of quantumconfinement. In this manner, the thermoelectric efficiency of thethermoelectrically active material is enhanced. The thermoelectricallyactive material is also referred to herein as the “thermoelectricmaterial”. The cladding material preferably comprises a suitable glasssuch as a glass comprising an amorphous material having no long rangeordering of its constituent atoms.

One aspect of the invention involves a method for producing practicalthermoelectricity by developing quantum-confined nanowires capable ofexhibiting high ZT values. As explained hereinabove, the equation forthe thermoelectric figure of merit, Z, can be rendered dimensionless bymultiplying it by an absolute temperature, T, such as the temperature ofthe hot junction of the thermoelectric device. It follows that thedimensionless thermoelectric figure of merit, ZT=(S²*σ/k)*T, can be usedin the evaluation of the performance and energy conversion efficiency,of any thermoelectric material or device.

For nanowires of PbTe, if the bulk thermal conductivity (k) of PbTe isconsidered, the ZT factor at 750 K is still very high (i.e., ZT ofaround 2.0 or more) using ZT=(S²*σ/k)*T. ZT factors increase withtemperatures between about 300 K and 750 K. For PbTe-basedthermoelectric nanowires, the value of S²*σ tends to peak at a certainlevel with the ZT factors increasing with decreasing nanowire width.However, after a certain nanowire width is reached, ZT factors begin tofall with decreasing nanowire width. The PbTe-based nanowires describedherein may be easily tailored to exhibit n-type or p-type conduction,either by changing the stoichiometry of Pb and Te or by adding someminor components/impurities.

Numerous thermoelectric materials, including PbTe, are sensitive tooxygen, which can degrade thermoelectric performance. For this reason,it is advantageous to have such thermoelectric materials sealed off andprotected from oxygen contamination within the target environment range.Of course, a thermoelectric device is not commercially viable if itcannot withstand the elements and environment it is intended to functionunder.

Although PbTe is the preferred thermoelectric material, otherthermoelectric materials may be employed, such as Bi₂Te₃, SiGe, ZnSb,Zn₂2 and Cd0_(.8)Sb₃, without departing from the scope of the presentinvention. The thermoelectric material may initially be in anyconvenient form, such as granules or powder.

Once fiber-drawn nanowire cables were produced using the methodsdescribed above, the electrical conductivity (σ) and thermoelectricpower (S) were measured and the variation of the parameter, S²*σ, wasdetermined. The parameter, S²*σ, is determined experimentally,multiplied by the measurement temperature (in K) and divided by theknown thermal conductivity (k) to provide the ZT values of the nanowiresproduced by the present invention.

Testing of the glass cladding without embedded nanowires using the Vander Pauw 4-probe instrument showed that the sample was very resistivesuch that the instrument did not measure any conductivity. Similarly,the measurement of thermopower using a conventional method (e.g. byemploying the Seebeck coefficient determination system, marketed by MMRTechnologies, Mountain View, Calif.) did not produce any result onaccount of the high resistivity of the glass cladding. However, theelectrical conductivity and thermoelectric power of PbTe-embedded cableswas readily measurable, indicating that the measured values ofelectrical conductivity and thermoelectric power are attributable to thecontinuous nanowires along the length of the cable.

The preferred thermoelectric material for the nanowire cables of thepresent invention is PbTe because of its advantageous thermoelectricproperties and reasonable cost. Using the known bulk thermalconductivity value for PbTe, the calculated ZT ((S²σ/k)*T) factor at 750K is>2.5. The S²σ of PbTe exhibits a definite tendency to peak at acertain nanowire width. Given that the best known ZT factors for bulkPbTe is around 0.5, the resultant ZT factors of around 2.0 or more isconsidered to be significantly enhanced by quantum confinement. The ZTfactor increases with decreasing nanowire width until this maximum valueis reached, and then the ZT factor begins to decrease with furtherdecrease in nanowire width. As would be appreciated by those of skill inthe art, other thermoelectric materials having suitable thermoelectricproperties (e.g., Bi₂Te₃) may be employed without departing from thescope of the invention.

In accordance with the present invention, a maximum diameter of thenanowires is preferably less than approximately 200 nm, most preferablybetween approximately 5 nm and approximately 100 nm. In cases where thecross-section of the nanowires is not circular, the term “diameter” inthis context refers to the average of the lengths of the major and minoraxis of the cross-section of the nanowires, with the plane being normalto the longitudinal axis of the nanowires. Nanowires having diameters ofapproximately 50 nm to approximately 100 nm that may be prepared using amethod of drawing of a thermoelectric material in glass cladding, asdescribed hereinbelow.

The cables of the present invention preferably are manufactured toexhibit a high uniformity in diameter from end to end. According to someembodiments of the invention, the maximum diameter of the glass claddingmay vary in a range of less than approximately 10% over the length ofthe cable. For less precise applications, the diameter of the nanowiresmay vary in a larger range (e.g., 5-500 nm, depending on theapplication). Electrically, the glass is preferably several orders ofmagnitude more resistive than the thermoelectric material it is employedto clad. The cables are generally based on a semiconducting wire,wherein the doping and composition of the wire is primarily controlledby changing the composition of the thermoelectric material to yield awire that exhibits either p-type or an n-type thermoelectric behavior.Advantageously, the cables may be used to develop superiorthermoelectric devices in a cost-effective manner.

According to the invention, a method of drawing a thermoelectricmaterial in glass cladding involves drawing the glass-cladthermoelectric material to form individual fibers (or monofibers) ofthermoelectric materials, which are preferably about 500 microns indiameter or less. As would be appreciated by those of ordinary skill inthe art, the monofibers may have diameters greater than 500 micronswithout departing from the scope of the invention. Cable diameters maybe brought down to 5-100 nm by repeatedly drawing fiber bundles ofmonofibers, and the concentration of wires in a cross-section of thecable may be increased to ˜10⁹/cm² or greater. Such cablesadvantageously exhibit quantum confinement for providing enhancedthermopower generation efficiency.

The method of drawing a thermoelectric material in glass cladding mayfurther comprise bunching the cable together and redrawing several timesin succession to produce a multi-core cable comprising glass-cladthermoelectric fibers. By way of example, the material to forming thefibers of a cable may comprise PbTe or Bi₂Te₃. The resulting cablecomprises a multi-core cable having a plurality of individual fibersthat are insulated from each other by the glass cladding. A particularglass cladding may be chosen to contain a specific composition to matchthe physical, chemical, thermal and mechanical properties of a selectedthermoelectric material. The glass cladding is preferably several ordersof magnitude higher in electrical resistivity than the metal, alloy orsemiconductor material that forms the thermoelectric fibers. Suitablecommercial glasses for most applications include, but are not limitedto, pyrex, vycor and quartz glass.

According to a further aspect of the invention, the metal, alloy orsemiconductor material that forms the fibers is varied to render a cableeither n-type or p-type, such that individual cables may be used as then-type and p-type components of a thermoelectric device. The cables maybe induced to exhibit quantum confinement by reducing the thickness orthe diameter of the fibers to a predetermined range, thereby increasingthe efficiency of thermopower generation.

Method of Drawing a Thermoelectric Material in a Glass Cladding:

Referring to FIG. 1, vertical tube furnace 10 is employed to provideheat for drawing glass-clad thermoelectric fibers. In particular,vertical tube furnace 10 includes a central lumen 11 for receiving apreform 12 comprising a glass tube 14 that is sealed at an area ofreduced cross-section 18 to form vacuum space 20 that is at leastpartially filled with thermoelectric material 22. The furnace is used tomelt the thermoelectric material 22 and glass tube 14 in preparation forone or more drawing operations for producing glass-clad thermoelectricfibers 24.

With further reference to FIG. 1, vertical tube furnace 10 comprisesfurnace shroud 26, thermal insulation 28 and muffler tube 30. Suitablematerials for muffler tube 30 include conductive metals such asaluminum. Vertical tube furnace 10 further comprises one or more heatercoils 34 embedded therein. More precisely, heater coils 34 are disposedbetween muffler tube 30 and thermal insulation 28, and refractory cement38 is disposed between heater coils 34 and thermal insulation to directthe heat produced by heater coils 34 inwardly to form a hot zone 40within muffler tube 30. Heater coils 34 are provided with leads 44 thatmay be insulated using a ceramic insulator 48. Additionally, athermocouple probe 50 is provided for measuring the temperature withinhot zone 40, which may include a length of approximately one inch.

A method of drawing a thermoelectrically active material 22 comprisingan array of metal, alloy or semiconductor rods embedded in a glasscladding will now be described. Initially, a suitable thermoelectricmaterial 22 is selected. The preferred thermoelectric material of thepresent invention comprises PbTe that is initially in granular form.Additional suitable thermoelectric materials include, but are notlimited to, Bi₂Te₃, SiGe and ZnSb. The next step involves selecting asuitable material for forming the glass tubing 14. The glass materialpreferably is selected to have a fiber drawing temperature range that isslightly greater than the melting temperature of the thermoelectricmaterial (e.g.,≧920° C. for PbTe). Vertical tubular furnace 10 is thenemployed to seal off one end of glass tubing 14. Alternatively, ablowtorch or other heating device may be used to seal off the glasstubing 14 and create vacuum space 20.

After sealing off one end of the glass tubing 14, the next steps involveintroducing the thermoelectric granules inside the vacuum space 20 andevacuating the tube by attaching the open end of the glass tube to avacuum pump. While the vacuum pump is on, an intermediate portion of theglass tubing 14 is heated such that the glass partially melts andcollapses under the vacuum. The partially melted glass tube provides anampoule 54 containing the thermoelectric material 22 to be used in afirst drawing operation. The next step involves introducing the end ofampoule 54 containing the thermoelectric material 22 into the verticaltube furnace 10. In the illustrated embodiment, the tubular furnace 10is configured such that the ampoule 54 is introduced vertically, whereinthe end of the ampoule 54 containing the thermoelectric granules isdisposed within hot zone 40 adjacent to heater coils 34.

Once the ampoule 54 is properly disposed in vertical tube furnace 10,the temperature is increased such that the glass encasing thethermoelectric granules melts just enough for it to be drawn, as is donein a conventional glass draw-tower, which is per se known in the art. Asdiscussed hereinabove, the composition of the glass is preferably chosensuch that the fiber drawing temperature range is slightly greater thanthe melting point of the thermoelectric granules. For example, if PbTeis selected as the thermoelectric material, pyrex glass is a suitablematerial for drawing the glass with PbTe fibers embedded therein. Thephysical, mechanical and thermal properties of glass tubing 14 andthermoelectric material 22 will have a bearing on the properties of theresulting cables. Glasses exhibiting a minimal deviation of theseproperties with respect to those of the thermoelectric material 22 arepreferably chosen as the cladding material.

The above-described glass tubing 14 may comprise commercially availablepyrex tubing having a 7 mm outside diameter and a 2.75 mm insidediameter, wherein the tube is filled with PbTe granules over a length ofabout 3.5 inches. Evacuation of glass tubing 14 maybe achieved overnightunder a vacuum of approximately 30 mtorr. After evacuation, the sectionof glass tubing 14 containing the thermoelectric material 22 is heatedgently with a torch for several minutes to remove some residual gas, andthen the glass tubing 14 is sealed under vacuum above the level ofthermoelectric material 22.

In operation, vertical tube furnace 10 is used for drawing theglass-clad thermoelectric fibers. Vertical tube furnace 10 includes ashort hot zone 40 of about 1 inch, wherein the preform 12 is placed inthe vertical tube furnace 10 with the end of the tube slightly below hotzone 40. With the furnace at about 1030° C., the weight from the lowertube end is sufficient to cause glass tubing 14 to extend under its ownweight. When the lower end of glass tubing 14 appears at the loweropening of the furnace, it may be grasped with tongs for hand pulling.Preform 10 may be manually advanced periodically to replenish thepreform material being used up during the fiber drawing process. Fiber24 preferably includes a diameter between about 70 microns and about 200microns. According to additional embodiments of the present invention,the drawing operation may be performed using an automatic draw-towerthat results in very little variation in diameter.

According to further embodiments of the invention, short fiber sectionsmay be formed by drawing the heterostructures and then breaking orcutting the heterostructures into shorter pieces. By way of example,these shorter pieces may be machined to be approximately 3 inches inlength. The pieces are then bundled inside another pyrex tube, which issealed at one end using the vertical tube furnace or using a blowtorch,as described hereinabove. When a suitable number of monofibers arepacked in the tube, the open end is attached to a vacuum pump and anintermediate section is heated. This heating causes the glass tube tocollapse, thereby sealing the tube and forming an ampoule for a seconddrawing operation, which produces a cable having a plurality ofmulti-core fibers. After the second drawing operation, the fibers arecollected and placed in the bore of yet another sealed tube. When thebore is filled with a suitable number of monofibers, the preform isevacuated and sealed under vacuum. Fiber drawing is then performed onthe twice-drawn fibers. This process is repeated as needed to obtain afinal thermoelectric material diameter of about 100 nm.

Nanowire Properties:

In order to characterize the electronic properties of bulk andheterostructure nanowires, it is important to determine the x-raydiffraction characteristics of the glass-clad thermoelectric material.FIG. 2 depicts an x-ray diffraction pattern of a PbTe-based cableconstructed in accordance with the principles of the present invention,wherein the characteristic spectrum of PbTe is overlaid on a glassyx-ray diffraction pattern. In particular, the x-ray diffraction patternclearly indicates the presence of PbTe peaks and a lack of other peaks,thus illustrating that the glass material has neither reacted with PbTenor devitrified during fiber drawing. These peaks are exclusivelycharacteristic to those of PbTe crystals.

FIG. 3 depicts a glass-clad PbTe-based cable 60 constructed using themethod of drawing a thermoelectrically active material embedded in aglass cladding described hereinabove. Specifically, the cable 60comprises a plurality of multiple monofibers 64 that are bundled andfused to form a cable (or button) of virtually any length. This buttoncan be broken, cut or otherwise sectioned to produce a plurality ofshorter cables having a predetermined length. FIG. 4 is an enlargedcross-sectional view of the glass-clad PbTe-based cable 60 of FIG. 3taken along line 3A-3A. Cable 60 includes a plurality of monofibers 64,has a width of approximately 5.2 mm, and was produced using a singledrawing of the PbTe fibers at a temperature of approximately 300K.

According to the preferred embodiment of the invention the cable 60 isbunched together and redrawn several times in succession to produce amulti-core cable having a plurality of individual thermoelectric fibersthat are insulated from each other by the glass cladding. FIG. 5 is across-sectional view of the glass-clad PbTe-based cable 60 after asecond drawing of the PbTe fibers. The twice-drawn cable has a width ofapproximately 2.78 mm. FIG. 6 is a cross-sectional view of theglass-clad PbTe-based cable 60 after a third drawing of the PbTe fibers,wherein the cable has a width of approximately 2.09 mm.

FIGS. 3-6 illustrate the development of microstructure as theconcentration of wires in the cable increases to ˜10⁹/cm². Thesemicrostructures may be observed using optical and scanning electronmicroscopes. By way of example, energy dispersive spectroscopy may beemployed to unambiguously indicate the presence of PbTe wires in theglass matrix.

Thermoelectric Property Characterization:

Another aspect of the present invention involves the continuity andelectrical connectivity of the glass embedded fibers along the entirelength of the cable. Electrical connectivity is easily demonstrated bydetermining the resistance of the cable at different thicknesses.According to a preferred implementation of the invention, the resistanceof the glass cladding, without any thermoelectric wires embeddedtherein, is about 7 to 8 orders of magnitude higher than that of thecontinuous thermoelectric fibers.

The samples used to determine electrical connectivity of thethermoelectric wires are in the form of “buttons” of PbTe prepared fromthe preforms following the one of the fiber drawing steps. Referring toFIGS. 7-9, the resistance of the thermoelectric wires embedded in theglass is approximately 1 ohm or less. On the other hand, the resistanceof the glass cladding without thermoelectric wires is more than 10⁸ohms, which is about 8 orders of magnitude higher than that of thePbTe-embedded cables. This difference in electrical resistance indicatesthat the glass-clad thermoelectric wires drawn using the methodsdescribed herein exhibit electrical connectivity from one end to theother.

FIG. 7 is a chart illustrating the DC resistance of PbTe cable 60 afterthe first drawing of the PbTe fibers, wherein the resistance of thecable (Ohms) is plotted against the electrical current (amps). Inparticular, the DC resistance of the cable 60 steadily decreases with anincreasing current. FIG. 8 is a chart illustrating the DC resistance ofthe cable 60 after the second drawing of the PbTe fibers, while FIG. 9is a chart illustrating the DC resistance of the PbTe cable 60 after thethird drawing of the PbTe fibers.

A preferred cable produced in accordance with the principles of thepresent invention preferably comprises at least one thermoelectric fiberembedded in an electrically insulating material, wherein thethermoelectric material exhibits quantum confinement. According to thepreferred embodiment of the invention, a width of each fiber issubstantially equivalent to a width of a single crystal of thethermoelectric material, wherein each fiber has substantially the samecrystal orientation. The preferred cable comprises a plurality of fibersthat are fused or sintered together such that there is electricalconnectivity between all the fibers. Alternatively, there is electricalconnectivity between some, but not all of, the fibers of the cable.

The glass cladding for the cable preferably comprises an electricallyinsulating material comprising a binary, ternary or higher componentglass structure such as pyrex, borosilcate, aluminosilicate, quartz, andlead telluride-silicate. The thermoelectric material may be chosen fromthe group consisting of a metal, a semi-metal, an alloy and asemiconductor, such that the thermoelectric material exhibits electricalconnectivity and quantum confinement along a predetermined length ofcable from several nanometers to miles. The ZT factor of the cable ispreferably at least 0.5, more preferably at least 1.5, most preferablyat least 2.5.

Thus, it is seen that methods of drawing high density nanowire arrays ina glassy matrix are provided. One skilled in the art will appreciatethat the present invention can be practiced by other than the variousembodiments and preferred embodiments, which are presented in thisdescription for purposes of illustration and not of limitation, and thepresent invention is limited only by the claims that follow. It is notedthat equivalents for the particular embodiments discussed in thisdescription may practice the invention as well.

1. A method of drawing a thermoelectrically active material in a glasscladding, the method comprising the steps of: sealing off one end of aglass tube such that the tube has an open end and a closed end;introducing the thermoelectrically active material inside the glass tubeand evacuating the tube by attaching the open end to a vacuum pump;heating a portion of the glass tube such that the glass partially meltsand collapses under the vacuum such that the partially melted glass tubeprovides an ampoule containing the thermoelectric material to be used ina first drawing operation; introducing the ampoule containing thethermoelectrically active material into a heating device; increasing thetemperature within the heating device such that the glass tube meltsjust enough for it to be drawn; and drawing fibers of glass cladthermoelectrically active material such that the thermoelectricallyactive material exhibits quantum confinement.
 2. The method of claim 1,further comprising the steps of: bunching the fibers of glass cladthermoelectrically active material together; and redrawing one or moretimes in succession to produce a multi-core cable having a plurality ofindividual thermoelectric fibers that are insulated from each other bythe glass cladding.
 3. The method of claim 1, further comprising thesteps of: breaking the glass clad fibers into shorter pieces;introducing the pieces of glass clad fibers into another glass tubehaving a sealed end and an open end; evacuating the tube by attachingthe open end to a vacuum pump; heating a portion of the glass tube suchthat the glass partially melts and collapses under the vacuum such thatthe partially melted glass tube provides an ampoule containing thepieces of glass clad fibers; introducing the ampoule into a heatingdevice; increasing the temperature within the heating device such thatthe glass tube melts just enough for it to be drawn; and drawing fibersof glass clad thermoelectrically active material to produce a cablehaving a plurality of multi-core fibers.
 4. The method of claim 1,wherein the heating device comprises a vertical tube furnace including acentral lumen for receiving the glass tubing.
 5. The method of claim 4,wherein the central lumen includes a hot zone for melting the glasstubing.
 6. The method of claim 1, wherein the thermoelectrically activematerial comprises an array of metal, alloy or semiconductor rodsembedded in the glass cladding.
 7. The method of claim 1, wherein thethermoelectrically active material comprises PbTe that is initially ingranular form.
 8. The method of claim 1, wherein the thermoelectricallyactive material comprises Bi₂Te₃, SiGe or ZnSb.
 9. The method of claim1, wherein the glass cladding is selected to have a fiber drawingtemperature range that is slightly greater than the melting temperatureof the thermoelectric material
 10. The method of claim 1, wherein theglass cladding comprises an electrically insulating material selectedfrom the group consisting of: pyrex; borosilcate; aluminosilicate;quartz; and lead telluride-silicate.
 11. The method of claim 1, whereinthe fibers have a diameter between 70 microns and 200 microns.
 12. Themethod of claim 1, wherein an electrical resistance of thethermoelectric fibers embedded in the glass is less than 1 ohm.
 13. Themethod of claim 1, wherein an electrical resistance of the glasscladding without the thermoelectric fibers is more than 10⁸ ohms. 14.The method of claim 1, wherein a width of each fiber is substantiallyequivalent to a width of a single crystal of the thermoelectricallyactive material.
 15. The method of claim 1, wherein the glass-cladfibers are fused or sintered together such that the fibers areelectrically connected end-to-end, but there is no lateral electricalconnection between the fibers.
 16. The method of claim 1, wherein theindividually fibers are fused or sintered together such that there isend-to-end electrical connectivity between some, but not all of, thefibers.
 17. The method of claim 1, wherein a ZT factor of the cable isat least 0.5.
 18. The method of claim 1, wherein a ZT factor of thecable is at least 1.5.
 19. The method of claim 1, wherein a ZT factor ofthe cable is at least 2.5.